1. Field of the Invention
The present invention relates to a battery management apparatus for grasping a change in impedance of a battery caused by the deterioration of the battery to reflect the change in battery management.
2. Description of the Related Art
In the field of vehicles, such as automobiles, to encourage low emission and resource saving, the development of hybrid electric vehicles is advancing. Emissions from hybrid electric vehicles are substantially lower than vehicles using a gasoline-powered engine and the hybrid electric vehicles ensure resource saving. A hybrid electric vehicle mounts a gasoline-powered engine and a motor which generates driving power using electric power supplied from a battery and operates using both the engine and the motor. In the above-mentioned hybrid electric vehicles, grasping the accurate battery state to manage the battery is of importance. For this purpose, the remaining capacity of the battery is calculated in addition to basic parameters, such as voltage, current, and temperature. The remaining capacity is affected by the deterioration of the battery. Disadvantageously, it is difficult to maintain high accuracy with which to calculate the remaining capacity over the long term.
Japanese Unexamined Patent Application Publication No. 6-242193 (Patent Document 1) discloses a technique of obtaining the remaining capacity of a battery on the basis of its open circuit voltage obtained from the battery voltage measured when an electric vehicle stops, detecting the discharged capacity based on the integrated value of the discharge current of the battery, calculating the fully-charged capacity on the basis of the discharged capacity and the remaining capacity at the stop time, and calculating the degree of deterioration on the basis of the fully-charged capacity and the nominal fully-charged capacity, thus detecting the remaining capacity in consideration of the degree of deterioration.
According to the technique disclosed in Patent Document 1, the open circuit voltage is obtained based on the battery voltage measured when the vehicle stops. Disadvantageously, the open circuit voltage is obtained with no consideration given to the following fact: As the battery deteriorates, the internal impedance thereof increases, so that the open circuit voltage is affected. In addition, in electric vehicles, even when its motor stops, current flows in a load, e.g., an inverter. Accordingly, the accurate open circuit voltage is not always detected. Therefore, the technique of Patent Document 1 is of limited application. It is insufficient to manage the state of a battery according to various mounting conditions.